1. Field of the Invention
The present invention relates to a niobium solid electrolytic capacitor and its production method.
2. Description of the Related Art
Niobium has a dielectric constant about 1.8 times larger than that of tantalum which is a material of conventional solid electrolytic capacitors, and is therefore specifically noted as a material of next-generation high-capacity solid electrolytic capacitors.
However, when a solid electrolytic capacitor is mounted on the surface of a substrate, it may be exposed to high heat in are flow process. In a niobium solid electrolytic capacitor, a part of oxygen in the dielectric layer of niobium oxide may diffuse into the anode, whereby the thickness of the dielectric layer may reduce and some defects may form in the dielectric layer. As a result, the dielectric layer may give leak current and the layer may peel from the cathode owing to the defects, thereby causing a problem in that the capacitance may reduce.
For preventing the characteristic deterioration in the reflow process of mounting a solid electrolytic capacitor, investigated is a method of adding niobium oxide to a sintered anode body (JP-A 2002-231583, and WO2004/068517).
For the purpose of reducing leak current, proposed is a method that comprises processing an anode of niobium or a niobium alloy for anodic oxidation in an aqueous solution containing a fluoride ion followed by further anodic oxidation in an aqueous solution containing a phosphate ion or a sulfate ion (JP-A 2005-252224).
Also proposed is nitridation of anode for reducing the capacitance change before and after the reflow process in part-mounting a niobium solid electrolytic capacitor (JP-A 11-329902).